Effects of local non-equilibrium solute diffusion on rapid solidification of alloys

A conceptual foundation for the study of local non-equilibrium solute diffusion under rapid solidification conditions is proposed. The model takes into account the relaxation to local equilibrium of the solute flux and incorporates two diffusion speeds, V, the bulk liquid diffusion speed, and V, the interface diffusive speed, as the most important parameters governing the solute concentration in the liquid phase and solute partitioning. The analysis of the model predicts complete solute trapping and the transition to a purely thermally controlled solidification, which occur abruptly when the interface velocity V equals the bulk liquid diffusion speed V. The abrupt change in the solidification mechanism is described by the velocity dependent effective diffusion coefficient D* = D(1 - V/V) and the generalized partition coefficient K*. If V > V then D* = 0 and K* = 1. This implies an undisturbed diffusion field in the liquid (diffusionless solidification) and complete solute trapping at V > V. The interface diffusion speed V governs solute redistribution at a relatively low interface velocity V ∼ V < V. The influence of the local non-equilibrium solute transport on the temperature field and interface stability under rapid solidification conditions is also discussed.